Fix reported regex and struct text signatures - #9534

Summary

Fix 13 signature mismatches reported by whats_left --signature in #9530: six re.Pattern methods, two bound aliases in fractions, and five _struct APIs.

• Render the existing StringPos(0) default through typed metadata; preserve integer conversion, negative-position clamping, and endpos=sys.maxsize.
• Name packing varargs values and module format parameters format.
• Derive Struct(format) from its typed initializer, retaining the implicit-initialization bypass before argument binding in slot_init.
• Add three pure Rust tests against generated native docs (seven regex methods including the match alias, Struct, and four packing functions).
• Reuse Align builtin signatures exposed by whats_left #9530's one-line CI --signature output flag. No new scan, import, or build step.

Reference: CPython 3.15.0 d220409a13b77de7a6c30abe16ea90db01528edc, Modules/_sre/clinic/sre.c.h, Modules/clinic/_struct.c.h, and _struct.c::s_init.

Verified results

At head 9f565fc8c91a1efcdd3ab5630993cd6a325930b5, CI run 41454455490 completed successfully: all 24 jobs passed. CodSpeed also passed; pyperformance was skipped by its existing conditions.

Same-platform normalized inspect.signature mismatches versus #9530's report-only head 68646843423633e60bd54c6d4150db953b303804:

Each platform removes exactly the same 13 targeted entries. This compares normalized signatures, not raw text alone.

• All three new native metadata tests passed on all three operating systems.
• All three native runtime jobs passed, including existing test_re, test_struct, test_fractions, and test.test_inspect.test_inspect suites.
• All Rust, clippy, WASM, cross-target, Miri, docs, and lint/zizmor jobs passed. Separate Linux/macOS C-API test results were 129 passed, 4 ignored each. Existing skips/ignores remain; no no-skip claim is made.
• Local configured hooks and git diff --check passed. Native compilation/runtime validation was performed by CI, not locally.
• CI caught a test-only trait-owner compilation error in the first head. Follow-up commit 9f565fc8 selects the Pattern payload's generated metadata; every assertion remains intact. The corrected head above is fully green.

Integration note: the NameValues export shares an import-list hunk with the concurrent builtin-signature cohort's NameObjects; retain both names when combining.

AI assistance

Implementation, tests, source analysis, CI investigation, and this description were AI-assisted using OpenAI Codex (model version not exposed). No Lib tests, assertions, or snippets were changed.